With the recent widespread proliferation of digital cameras and personal computers, silver halide photographic light-sensitive materials have been used as print materials of digital image information with growing frequency. As to the print materials of digital image information, recent years have also seen widespread use of image output materials other than silver halide photographic light-sensitive materials, typified by inkjet printers. To compete against these output materials, the field of silver halide photographic light-sensitive materials including color photographic paper, has been ever more strongly required to increase the speed of photographic processing, to enhance image quality, and to improve processing stability (consistency).
Further, in recent years, digitalization has been remarkably propagated also in the field of a color print using a color photographic paper. For example, a digital exposure system by laser scanning exposure has been rapidly spread in comparison with a conventional analog exposure system of directly conducting a printing from a processed color negative film using a color printer. The digital exposure system is characterized in that a high image quality is obtained by conducting image processing, and it greatly contributes to improvement of qualities of color prints using a color photographic paper. Further, it is also considered to be an important factor that a color print with a high image quality is easily obtained from these electronic recording media such as digital cameras. It is believed that they will lead to further remarkable popularization.
On the other hand, as a color print method, techniques, such as an ink jet method, a sublimated type method, and color xerography have each progressed and are recognized for their ability of providing comparable image qualities to photography. Compared with these techniques, characteristics of the digital exposure method using color photographic paper reside in high image quality, high throughput, and high solidity (fastness) of an image. It is desired to further develop these characteristics and to provide high image quality photographs more easily and with lower cost. If so-called one-stop service of a color print becomes possible (i.e., one shop receives a recording medium of a digital camera from a customer and finishes processing, to return a high image-quality print to the customer in a short time such as a few minutes), the predominance of the color print using color photographic paper will further increase. If rapid processing suitability of color photographic paper is raised, a printing apparatus which is smaller in size and lower in costs while having high productivity, can be used, and thus the one-stop service of a color print is expected to spread further. From these points, in particular, it is important to raise the rapid processing suitability of color photographic paper. To realize the one-stop service of the color print using color photographic paper, analyses from various viewpoints, such as shortening of exposure time, shortening of so-called latent image time from the exposure to the initiation of the processing, and shortening of time period from the processing to the drying are required. Thus, conventionally, various kinds of proposals have been proposed based on such viewpoints.
Silver halide emulsions for use in color photographic paper must meet various requirements as mentioned above. As such silver halide emulsion for use in color photographic paper, a silver halide emulsion of a high silver chloride content has been used primarily because of a demand for rapid processing.
In addition, it is desirable for further rapid processing that silver halide emulsions be reduced in grain size. Especially, there is an intense demand on color photographic paper, to reduce the grain size of a yellow-dye-forming silver halide emulsion, which has the greatest grain size of all the silver halide emulsions in the color photographic paper, and the grain size of a silver halide emulsion in the emulsion layer nearest to the support, which emulsion is slow in progress of development. As to spectrally sensitized emulsions, grain size reduction generally results in lowering of sensitivity, because the emulsion sensitivity is proportional to the surface area of silver halide grains. Aimed at further increasing the sensitivity, therefore, various improvements have been made to methods of chemical sensitization and methods for forming silver halide emulsion grains.
Representatives of known methods for chemically sensitizing silver halide emulsions are sulfur sensitization, selenium sensitization, tellurium sensitization, precious-metal sensitization, including gold sensitization; reduction sensitization, and combinations of two or more of these. To mention selenium sensitization, in particular, of those sensitization methods, it is known that selenocarboxylic acid esters, i.e. seleno esters, are usable as selenium sensitizers (e.g. in U.S. Pat. Nos. 3,297,446 and 3,297447, and JP-B-57-22090 (“JP-B” means examined Japanese patent publication)).
Although there are cases in which selenium sensitization can produce a greater sensitization effect than sulfur sensitization generally performed in the field, selenium sensitization tends to cause heavy fogging and soft gradation. In addition, the combined use of selenium sensitization and gold sensitization can bring about a remarkable increase in sensitivity, but at the same time it causes increased fogging and tends to enhance soft gradation. Therefore, there has been a strong need for development of selenium sensitization methods capable of ensuring reduced fogging and hard gradation.
JP-A-7-140579 (“JP-A” means unexamined published Japanese patent application) proposes chemical sensitization of silver chloride emulsions by use of selenium compounds, aiming to increase the sensitivity and reduce fogging when the emulsions undergo 1/10-second exposure. Such a chemical sensitization method is too high in fog density to be applied to color photographic paper, and therefore development of methods capable of achieving further reduction in fogging and improvement in sensitivity is expected.
On the other hand, specified disulfide compounds, sulfinate compounds, combinations of specified disulfide compounds and sulfinate or seleninate compounds, and combinations of specified selenium compounds and radical scavengers are proposed in European Patent No. 627657, JP-A-6-35147, JP-A-6-202265, and JP-A-10-31279. However, these references are silent on the fogging problem in selenium sensitization.
Further, when silver halide emulsions having high silver chloride contents undergo high-illumination exposure, such as laser exposure, there results a sensitivity reduction and soft gradation enhancement, or so-called high illumination intensity reciprocity law failure. It is known that such failure can be improved by incorporating metal complexes into silver halide grains (as disclosed, e.g., in U.S. Pat. No. 5,360,712).
However, in the case of enhancing hard gradation by use of metal complexes, there is a requirement to enhance the hard gradation characteristics under conditions of low illumination intensity, in addition to enhancing the hard gradation characteristics under conditions of high illumination intensity by the foregoing improvement of reciprocity law failure. When metal complexes are used to satisfy those requirements, a desensitization problem arises, and latent-image stability becomes insufficient.
Arts of using selenium (Se) sensitization compounds in silver halide emulsions having high silver chloride contents are known (e.g. in JP-A-2003-287838). However, such arts are insufficient to ensure high sensitivity, hard gradation, and latent-image stability, regardless of what the illumination intensity of exposure may be (i.e. in any of illumination intensities ranging from low illumination intensity to high illumination intensity). As such, those arts are in need of improvements.
JP-A-4-335336 proposes the art of improving reciprocity characteristics, latent-image stability, and pressure immunity by using an emulsion that has a high silver chloride content, is incorporated a metal complex having at least two CN ligands, and further is subjected to selenium sensitization. However, metal complexes having many CN ligands are unsuccessful at imparting satisfactory hard gradation characteristics under conditions of low illumination intensity to the emulsions. JP-A-6-308652 proposes improvement of storage stability by emulsions having high silver chloride contents, containing metal complexes, and further, having undergone selenium sensitization. However, this reference provides no suggestion about the art of improving the desensitization caused by metal complexes used for obtaining hard photosensitive materials.
Further, there are suggestions that performances can be improved by incorporating small amounts of silver bromide and/or silver iodide into emulsion grains having high silver chloride contents. For instance, it is suggested that high sensitivity can be obtained by localizing phases having high silver bromide content in a wide variety of forms in emulsion grains having high silver chloride contents (U.S. Pat. No. 5,399,475); and that emulsions increased in sensitivity and reduced in high illumination intensity reciprocity law failure can be obtained by incorporating iodide into emulsions high in silver chloride content so as to have a maximum iodide concentration at a sub-surface of the emulsion grains (e.g. U.S. Pat. Nos. 5,726,005 and 5,736,310); and that emulsions having excellent performances with respect to reciprocity law failure, temperature dependence at the time of exposure, and pressure immunity can be obtained by incorporating specified compounds into grains having iodide bands formed at the time when 93% of grain formation is completed (EP 0928988A).
On the other hand, there is a proposal to impart super-hard photographic characteristics, developability at low pH, and satisfactory storability to black-and-white silver halide light-sensitive materials, by using a combination of a metal complex, such as a Rh complex, and a selenium or tellurium compound having a specific structure, in photosensitive materials that can form halftone dot images for printing (graphic arts) (JP-A-8-95184). However, the reference does not apply such a combination to a silver halide color photographic light-sensitive material, and the adoption of the method specifically described in JP-A-8-95184 becomes a cause of considerable desensitization.